tls: rx: factor out writing ContentType to cmsg

cmsg can be filled in during rx_list processing or normal
receive. Consolidate the code.

We don't need to keep the boolean to track if the cmsg was
created. 0 is an invalid content type.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>